Successful purification of a liquid or gaseous argon requires removal of low concentration (i.e., in the range of parts per million) of oxygen from argon. The removal of up to 1 percent of oxygen from argon is considered to be a purification process and is necessary for many end users of argon where the presence of oxygen in the argon is undesirable. In many instances where safety, handling, and the industrial or laboratory use of argon in either a liquid or gaseous state occurs, the purity of argon is important. Argon is colorless, odorless, and nontoxic as a solid, liquid, and gas. Argon is chemically inert under most conditions. As an inert noble gas, it possesses special properties desirable for applications related to the semi-conductor industry, for lighting, welding and other high-temperature industrial processes where ordinarily non-reactive substances can become reactive. Oxygen, in contrast to argon, is a highly reactive substance (in gaseous or liquid form) and is often a safety concern in that it supports combustion. Even low levels of oxygen (<100 parts per million) are many times not acceptable for certain laboratory and industrial processes. This also includes the chemical processing industry where certain reactions must be carried out, primarily in the absence of oxygen.
Cost considerations for the purification of argon have been a driving influence in the development of special cryogenic systems over at least several decades, and finding the proper process which is robust, reliable, and meets the economic criteria necessary to meet customer demand at an acceptable price has been challenging. These challenges have been the focus of several other investigators. Production of liquid argon via cryogenic distillation is well known and is the preferred method for producing high purity argon.
Adsorption processes can also be used to achieve the required purification and are reported in the literature. However, most of the related art is focused on the purification of gaseous streams. Liquid argon (rather than gaseous argon) is more easily transported to the customer location and once the customer receives the liquid argon, the conversion to the gaseous state is easily achieved. Therefore, it is desirable to keep argon in the liquid form during and after the purification process. Finding a suitable adsorbent to accomplish this task, primarily in the liquid phase, is one major focus of this disclosure.
In the related art, U.S. Pat. No. 3,996,028 describes a VPSA process for the purification of argon from oxygen. The argon is passed through a bed of synthetic zeolites of the A type and the oxygen is adsorbed on the zeolite. Thereafter, regeneration is caused by decreasing the bed pressure and ultimately using vacuum. This patent claims the use of any form of zeolite A with an entry void diameter of 2.8 to 4.2 Å (angstroms). According to this document, ion exchanged forms of zeolite A can be used to decrease the entry void diameter and in addition allows for altering the working temperature and pressure range of the purification process. There is no teaching in the disclosure regarding the benefit of varying the entry void diameter within the range of 2.8 to 4.2 Å other than the ability to operate under different temperature and pressure conditions. Moreover, it is not clear how the entry void diameter parameter was determined. One skilled in the art will recognize that this parameter is not simple to measure. Ion exchanged zeolite A compositions are also disclosed which have entry void diameters within the 2.8 to 4.2 Å claimed range including, greater than 10 percent lithium ion exchanged zeolite A. The disclosure also does not describe a maximum ion exchange level for lithium within the adsorbent, and does not describe nor define the combined importance of creating adsorbent compositions having the proper oxygen capacity, argon capacity, and oxygen to argon selectivity for purification purposes.
U.S. Pat. No. 4,477,265 describes the recovery of argon from a gas stream containing nitrogen and oxygen by passing the gas stream through a first bed containing an adsorbent with equilibrium selectivity for nitrogen and subsequently passing the stream through a physically separate adsorbent bed using kinetic selectivity for oxygen adsorption. This document describes the use of a carbon molecular sieve, as the preferred adsorbent, with a selectivity favoring oxygen over argon. While there is no selectivity at true equilibrium, this disclosure proposes using a short contact time of the gas with the adsorbent. This would allow oxygen to adsorb onto the adsorbent, but provide insufficient time for the argon to adsorb.
U.S. Pat. No. 5,159,816 describes a process for preparing high purity argon using cryogenic adsorption by removal of nitrogen and oxygen using a molecular sieve suited to physical nitrogen adsorption and likewise a molecular sieve suited to the physisorption of oxygen. Concerning the adsorbent selection, it is claimed that 4A zeolite should be used in the adsorbent bed for oxygen removal from argon, and a 5A molecular sieve be used for removal of nitrogen. In the specification, representative molecular sieves for nitrogen and oxygen are identified as 5A, 4A, Mordenite, 13X, Chabazite, Erionite and ion exchanged variants using cations other than Na, including K, Li and Ca. In the subject disclosure, no specific adsorption characteristics for oxygen and argon are taught. Additionally, the relationship between performance, adsorbent type, and composition is neither described nor discussed in this patent, but instead a very diverse list of zeolite structures and cation types are claimed.
U.S. Pat. No. 5,601,634 describes a process for removing nitrogen and oxygen from an argon stream. The patent describes a two-step process where one type of adsorbent is used to remove nitrogen using a temperature swing adsorption process (TSA), and a second bed is then employed to remove oxygen. The oxygen removal bed, utilizes carbon molecular sieve (CMS) or 4A type zeolite. The adsorbent characteristics for effective oxygen removal from argon, as taught in the present disclosure are not met by Zeolite 4A. The broad pore size distribution of a CMS is expected to rule out the use of this adsorbent for the required process described herein. In comparison with U.S. Pat. No. 4,477,265, discussed above, short cycles were necessary for CMS, whereas the high performance of the adsorbents of the present invention enable long cycle times to be achieved.
U.S. Pat. No. 5,784,898 describes a process for the preparation of a fluid, including liquid argon which is purified from impurities—including oxygen. The adsorbent is selected from a group consisting of various natural and synthetic zeolites, optionally ion exchanged with different cations, and porous metal oxides. Hopcalite, a mixed metal oxide, is specifically identified as being effective for purifying carbon monoxide and oxygen simultaneously. This document demonstrates a lack of definition regarding a preferred composition or range of compositions. Instead, all known natural and synthetic zeolites are claimed, and in addition, porous metal oxides are included.
U.S. Pat. No. 5,685,172 describes a process for the purification of oxygen and carbon dioxide from a cold gas or liquid stream of at least 90 mol percent of nitrogen, helium, neon, argon, krypton, xenon, or a mixture of these gases. To achieve this, the use of a porous metal oxide, such as hopcalite-like materials are required. The regeneration of these metal oxides requires a reducing agent, such as hydrogen, which increases the total operating cost of adsorption processes using these materials. The zeolites described in the present invention are structurally, compositionally and functionally different to hopcalite and do not require use of reducing agents for regeneration. More specifically, hopcalites are chemisorbents or catalysts whereas zeolites, however, are reversible physical adsorbents. In addition, hopcalite materials are largely non-crystalline. Any crystallinity associated with hopcalite comes from the MnO2 component, which is present mainly in the amorphous form. In contrast, zeolites are crystalline materials.
U.S. Pat. No. 6,083,301 describes a PSA or TSA process for purifying inert fluids to at most 1 part per billion impurities for use in the field of electronics. This document describes the use of a hopcalite-like adsorbent for the capture of oxygen impurities from liquid streams.
Drawbacks associated with the related art include the use of hopcalite-like chemisorbents or catalysts that require the use of hydrogen as a reducing agent, which are costly, and do not possess the required physio-chemical properties needed for simple adsorbent regeneration. The adsorbents of the present invention are much easier to regenerate. In cases where full scale commercial argon purification is needed, affordable capital expenditures are required for the purification process. The purification level achieved using the adsorbent compositions of the present invention, is typically sufficient and acceptable for the majority of argon end-users.
The advantages taught in the present invention include using a superior crystalline microporous solid with a high oxygen capacity and an argon capacity that has been engineered to be as low as possible. This adsorbent would enhance the separation process versus the broader pore size distribution inherent for amorphous carbon. Moreover, the adsorption performance for the removal of oxygen from argon is closer to that of molecular sieving, than kinetic separation. As a result, process cycle times (or gas contact times) can be extremely long (i.e. 7 days or more depending on the feed concentration and process conditions) which is advantageous from the standpoint of economic feasibility.
In short, there are several limitations associated with the commercial purification of argon using adsorption compositions and techniques that have been discussed in the related art for certain applications. Additionally, we have determined missing information or data that was never known or published in the past, for example, in the '028 patent described above. According to the present invention and the argon adsorption uptake kinetics and capacity tests performed on lithium exchanged 4A zeolites, it is shown that samples with high lithium exchange levels (i.e. greater than or equal to 88 percent) possess an argon uptake rate and capacity well beyond that previously described and/or documented such that these adsorbents will not be effective in the purification. These known adsorbents and associated processes have been deficient in meeting all the criteria addressed above, namely: delivering argon as a liquid with very low oxygen concentrations in an economic, lower energy consuming process.
In summary, these previous adsorption compositions and related processes are not optimized for large scale operation in ASUs that produce up to a couple of hundred tons of liquid argon on a daily basis. Unmet needs remain regarding large scale liquid argon purification with low parts per million levels (down to or below 1 part per million is desirable) of oxygen using adsorption technology that also includes the development of an optimal, economic, and effective adsorbent. This includes finding adsorbents with the maximum capacity for oxygen uptake and negligible uptake for argon, which specifically enables the use of smaller adsorbent beds and/or longer process cycle times.
To overcome the disadvantages of the related art, it is an object of the present invention to provide a novel argon purification adsorbent composition for use during the argon purification process. The adsorbent must also be capable of being effectively regenerated to remove most of the adsorbed oxygen, by warming with a nitrogen or argon purge to above cryogenic temperatures.